As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased. Among such secondary batteries is a lithium secondary battery exhibiting high energy density and operating voltage and excellent preservation and service-life characteristics, which has been widely used as an energy source for various electronic products as well as for the mobile devices.
Depending upon kinds of external devices in which secondary batteries are used, the secondary batteries may be used in the form of a single battery or in the form of a battery pack having a plurality of unit cells electrically connected to one another. For example, small-sized devices, such as mobile phones, can be operated for a predetermined period of time with the power and the capacity of one battery. On the other hand, a battery pack including a plurality of cylindrical batteries connected in parallel and in series to each other needs to be used in laptop computers or small-sized personal computers (PCs), because high power and large capacity are necessary for the laptop computers or the small-sized personal computers. The cylindrical batteries are preferable to prismatic batteries or polymer batteries in the aspects of capacity and power.
Therefore, a battery pack used in laptop computers or small-sized personal computers is configured to have a structure in which a core pack including a plurality of cylindrical batteries connected in parallel or in series to each other is mounted in a pack case in a state in which a protection circuit module (PCM) is connected to the core pack.
Meanwhile, in recent years, semi-middle-sized devices, such as camcorders, navigators and portable media players (PMPs), based on secondary battery packs, have been widely used which require higher power and larger capacity than small-sized devices, such as mobile phones, but lower power and smaller capacity than middle-sized devices, such as laptop computers.
As a power source to provide power and capacity required by such semi-middle-sized devices, a battery pack including a plurality of prismatic or pouch-shaped batteries, which are used in small-sized devices, connected to each other may be considered. In this case, however, energy density may be lowered due to the structure of the prismatic or pouch-shaped batteries.
Therefore, cylindrical batteries may be considered to be used as a power source for semi-middle-sized devices. However, conventional cylindrical batteries are optimized to be suitably used as unit cells of middle-sized devices. For this reason, it is required to greatly modify the structure of the batteries so that the batteries can be used as a power source for semi-middle-sized devices.
Consequently, there is a high necessity for a secondary battery pack based on a cylindrical battery suitable to be used as a power source for such semi-middle-sized devices.